Clarification of Terminology ???

Ok I feel pretty silly asking the following question, but they came up in a recent interview and I was not sure how to answer them.

Question: Can you please explain the difference between a Module Pool and a Sub Pool?

My answer: The Module pool is used in Dynpro programming to hold all the dialog modules PAI and PBO related to a particular group of screens. I am not aware of any objects within SAP called a sub pool.